An instrument used in measurement applications is typically required to be certified as calibrated to a standard that is traceable to international standards. Often the calibration must be carried out to a degree of accuracy necessitating the frequent recalibration of the instruments at a standards laboratory.
Instrument calibration is intended to eliminate or reduce bias in an instrument's readings over a range for all continuous values in the range. For this purpose, a reference standard with a known value for at least one selected point in the range of interest is measured with the instrument in question. Then a functional relationship is established between the value of the reference standard and the corresponding measurement by the instrument.
Generally during calibration at a standards laboratory, the instrument is calibrated using one of several in-house standards that are traceable to the international standards. These traceable in-house standards are adjusted to be consistent with more accurate standards in the calibration hierarchy periodically, or they are generated following an established procedure using other traceable physical quantity. An example of the latter is the generation of an in-house DC voltage standard using a traceable frequency and a Josephson junction array.
In recent years, the development of the time/frequency dissemination techniques has made it easier to access the standard time/frequency that is traceable to international standards. An example is the dissemination of the standard time/frequency using the global positioning system (GPS). The weighted average frequency of the atomic frequency standards on each of a number of GPS satellites is monitored and corrected by the GPS ground stations so that the weighted average frequency from a GPS satellite is traceable to the international frequency standard. Thus one is able to access a traceable time/frequency standard using a GPS receiver in combination with a suitable local oscillator, e.g., a stable quartz crystal oscillator or a rubidium frequency standard. Similarly, the growing prevalence of network time protocols allows the distribution of this time within a facility. For example, the protocol IEEE 1588, which is based on the teachings of U.S. Pat. No. 5,566,180, Eidson et al., entitled “Method for recognizing events and synchronizing clocks”, enables the transfer of time over Ethernet to accuracies approaching 1 ns or shorter.
In the international system (SI) base units, the time unit “second” has the smallest uncertainty, i.e., the time/frequency is the best measured physical quantity. Therefore, the unit “second” is used to define several other Si units such as the meter, volt, and ampere. Consequently, one is able to generate the required standard physical quantities using a traceable standard time/frequency. It would be advantageous to exploit the generation of such physical quantities for calibration purposes.